Rudder

Cavitation Formation of transient voids or vacuum bubbles in a liquid stream passing over a surface is called cavitation. This is often encountered arouna propellers, rudders, and struts and in pumps. When these bubbles collapse on a metal surface, there is a severe impact or explosive effec t that can cause considerable mechanical damage, and corrosion can be greatly accelerated because of the destruction of protective films. Redesign or a more resistant metal is generally required to avoid this problem. 

Stopa made the transition from sealing the rudder shaft in the bottom of a sailing vessel, to become braided packing that could resist the temperatures and pressures or steam and the high velocity shafts. 

The rudder is provided with anodes on both sides these should be fixed either at the level of the propeller hub or as far as possible above and below on the mdder blade. There are specially shaped rudder anodes which are welded to the front edge of the rudder. Sea chests and scoop openings should be specially considered and provided with anodes because of their increased current requirement. 

Fig. 17-4 Distribution of galvanic anodes for different shapes of stem, (a) Steamer stern one propeller, one balance rudder, (b) Steamer stern one propeller, a suspension rudder, (c) Tugboat stem two propellers, a Kort nozzle mdder. (d) Transom stem two propellers, a suspension mdder. (e) Transom stem two propellers, two suspension mdders.

Anodes are not attached to the rudder but are situated between the rudder shaft and the ship s wall and connected via a copper strip. The propeller is protected via a slip ring on the shaft. To achieve a low-resistance contact, the divided copper or bronze ring has a rolled silver-bearing surface on which metal graphite brushes slide. The transmission voltage should be below 40 mV. 

In the case of the ship shown in Figure 1.3, the rudder and engines are the control inputs, whose values can be adjusted to control certain outputs, for example heading and forward velocity. The wind, waves and current are disturbance inputs and will induce errors in the outputs (called controlled variables) of position, heading and forward velocity. In addition, the disturbances will introduce increased ship motion (roll, pitch and heave) which again is not desirable. 

The rudder provides a control moment on the hull to drive the actual heading towards the desired heading while the wind, waves and current produce moments that may help or hinder this action. The block diagram of the system is shown in Figure 1.13. 

Demanded rudder-angie Measured rudder-angie... 

A control system may have several feedback control loops. For example, with a ship autopilot, the rudder-angle control loop is termed the minor loop, whereas the heading control loop is referred to as the major loop. When analysing multiple loop systems, the minor loops are considered first, until the system is reduced to a single overall closed-loop transfer function. 

The training file eonsisted of input data of the form Time elapsed t kT), Rudder angle 6(kT), Engine speed n(kT) with eorresponding output data Forward veloeity u kT), Lateral veloeity v(kT), Yaw-rate r kT). 

With the engine speed held eonstant, the rudder was given step ehanges of 0°,... 

So, for example, with the ship model shown in Figure 10.26, the inverse model eould be trained with time, forward veloeity, lateral veloeity and yaw-rate as input data and rudder angle and engine speed as output data. 

Fig. 43. Grid-lock F-15 rudder fairing. Reproduced by permission of the Boeing Company.

Waterjets have been developed for application to high-speed ships. The waterjet has an inlet usually on the side or bottom of the ship in the region of the stern, which allows water to flow into a water pump. The pressure of the water is raised in the pump, and the water is expelled as a jet to produce the desired thrust. The direction of the jet flow can be controlled to provide maneuvering forces, eliminating the need for rudders. The propulsive coefficient of modern... 

Stern post rudder invented for sailing vessels. 

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